The mixed
propagation paths are considered for high frequency surface wave radar (HFSWR)
and radar path losses are estimated for homogeneous patches with different
electrical properties of sea and land as well as disturbed sea state influence.

1. Introduction. Long distance surveillance of 200 nautical
miles Exclusive Economical Zone (EEZ) using HFSWR is a developing technology
[1], which has already shown its capa­bilities to detect ships and aircrafts,
to collect data about the ocean environment remotely. The working principle of
HFSWR is based on the diffraction of HF radio waves with vertical polari­zation
along the earth surface [2]. There are different scenarios of HF surface wave
propagation depending on the region and HFSWR coverage, e.g. the propagation
along smooth sea surface is a homogeneous path, the propagation along disturbed
sea surface is a inhomogeneous path, the propagation along the sea surface with
many islands of different size is a mixed path. In the lat­termost case the
electric properties of the land can be inhomogeneous.

The radar equation
for HFSWR can be expressed via the signal-to-noise ratio qfor the working frequency fafter a
coherent integration of reflected target signals:

where Pis the energy potential, ais the
radar cross section of a target, Tcitis the coherent integration time, Wis the two-path radar attenuation, Lsysis the system losses, Nis the in­terference
level.

Following (1) the
main component of radar propagation losses is the radar attenuation W because there are several hardware and
software techniques to minimize these losses for the ac­tive and passive
interference level Nthat
influences the HFSWR operation. It is necessary to propose an adequate
propagation model in order to estimate the signal attenuation along the path
for different working frequencies. The final accurate solution hasn't been
found yet. The formulas in [2] are complicated to realize therefore the
Millington's empirical technique [3] seems to be simple and gives satisfactory
results compared to the theory in [2]. This technique is used in CCIR Rec.
368-8 [4] and includes the problems of homogeneous and mixed HF sur­face
propagation paths. Nevertheless the use of graphical data from [4] is not
convenient for the HFSWR permanently choosing the operating frequencies with
minimal active interference lev­els to adapt to complicated electromagnetic
environment. Such estimation is necessary in real time especially during the
HFSWR operation along the mixed paths.

To overcome this
problem there is a necessity to develop an interface-friendly software tool to
estimate radar path losses. The program is based on the program GRWAVE [4] to
calcu­late the field strength according to [5] and uses the mathematical model
of exponential atmos­phere CCIR. Actually this program is used to calculate the
filed strength of surface waves along the smooth homogeneous earth with the conductivity
aand the relative permittivity s. Mil-lington's technique is utilized to
analyze the radar losses estimation under the mentioned above environment.

2. Millington's technique for mixed paths. The surface wave propagation attenuation
between the transmitter and the receiver at the distance dcan be determined as [4]

PaTc

(1)

W(d)=10log10 (Pr/Pt),

(2)

where Pris the signal power at the receiver input, Ptis the transmitted power.

СРРСН'2008

The received power
from the short electrical dipole with the transmitting power

Pt=1kWatt can be defined using the field
strength E:

(3)

Pr(d)=E(d)222/(4nZ0) where Xis the wave length, Z0= 120n is
the impedance in free space.

Substituting (3)
in (2) the one-way propagation attenuation for the path consisted of several ho­mogeneous
patches is expressed as

W(d)= 142+20logf+ 20logE(d),(4)

where the working
frequency fis in MHz
and the filed strength Eis in
uV/m.

The recursive
equations of Millington's technique correspond to the plots in [4]

N , . N

ED =ZEk(sk)-ZEk(sk-1)sk =Zdn =d1 +d2 +d3 +...+dk

k=1 k=2

N N

ER = ZEk (rk
)-ZEk-1(rk ),

k=1k=2

where EDand ERare the filed strengths for direct path (transmitter-receiver) and
return path (receiver-transmitter) respectively, dnis the length of the n-th path patch. The path patches sk and rkare shown in Fig. 1.

k

Zdn

n=1

k

=ZdN-n+1= dN + dN-1+ dN-2+... + dN-k n=1

S4

Si

S2

S3

r5

Fig. 1. Different
patches of the mixed propagation path.

The total field
strength of the mixed propagation with several patches is equal to

Et =(Ed +Er )/2.

The recursive
equations of Millington's technique in case of two-patch propagation path can
be written as

ed=e1(d1)-e2(d2)+e2(d1+d2) , er=E2(d2)-e1(d2)+e1(d1+d2) ,

where the filed
value E1(d1), E1(d2), E2(d1), E2(d2), E1(d1+d2) and E2(d1+d2) are defined as following: E1(d1)is the field strength at distance for the homogeneous medium
1;

)is the field strength at distance d2for the homogeneous medium 1; E2(d^)- is the field strength at distance d-1for the homogeneous medium 2; E2(d-2)is the field strength at distance d2for the homogeneous medium 2; +d2) is the field strength at distance d-1+d2for the
homogeneous medium 1; E2(d1+d2) is the
field strength at distance d-1+d2for the
homogene­ous medium 2.

The similar
procedure continues in case of npatches of
the propagation path. 3. Radar losses modelling. Using the developed software the research is done to estimate radar
losses for the HFSWR in case of mixed propagation paths, i.e. there are islands
inside the

ri

r

r

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coverage zone, as
well as in case of additional losses due to disturbed sea surface. To estimate
HFSWR losses in case of propagation along the mixed paths with homogeneous
properties, sev­eral path scenarios are considered. Initially the propagation
scenario of the path "sea-land-sea-land-sea" (see Fig. 2, left) is
considered at patch lengths equal to 100 km, electric properties of sea surface
with a =5 S/m and s = 70 (average sea water salinity), electric properties of dry soil
with a =0.01 S/m and s = 15. The case of homogeneous propagation path along the smooth sea
surface is shown in the right part of Fig.2 by dashed lines. The case of mixed
propa­gation path including two islands is shown in Fig.2 by solid lines. The
electric properties of sea surface are a =5 S/m
and s = 80, the electric properties of land are a =0.003 S/m and s = 15. The
islands of 20 km and 50 km sizes are at the distances 50 km and 170 km
respectively. In Fig. 2 the homogeneous path (thick horizontal line) and EEZ's
border (vertical line) are shown.

Fig. 2. Two-path
surface wave attenuation while propagating along the mixed path. Left part:
using frequencies 5-25 MHz stepped by 5 MHz (the upper curve is at 5 MHz, the
lower curve is at 25 MHz). Right part: using frequencies 3.5 MHz, 5.5 MHz and
10-25 MHz stepped by 5 MHz (the upper curve is at 3.5 MHz, the lower curve is
at 25 MHz).

The Millington's
effect is seen clearly in these figures. The rapid increase of attenuation at the
border "sea-land" and the reconstruction at the border
"land-sea" show a capability to de­tect surface targets behind the
island. This effect has been already confirmed experimentally but it is still
necessary to investigate it further. Hereby it is important to notice that
islands increase the surface wave propagation attenuation; additional one-way
attenuation can be 15 dB due to mixed paths; the islands surrounding to the
HFSWR induce more attenuation compared to out­lying ones; the critical parameters
of mixed path attenuation are radial lengths of patches and distances between
them; additional attenuation can be minimized by HFSWR operating at lower
frequencies.

4. Disturbed sea surface influence on radar
losses. D. Barrick has
expanded his surface wave propagation theory by the case of rough sea surface
consideration [6], which is accepted to estimate the influence of disturbed sea
surface on the radar losses. In Fig. 3 the curves of addi­tional one-way
attenuation are shown at the frequencies 5 MHz (solid line), 10 MHz (dashed
line) and 20 MHz (dotted line) in case of different sea states, i.e. wind
speeds, and antenna loca­tions at sea level [6]. The additional attenuation A W increases according to increasing wind speed (or wave height),
working frequency and distance. For example, the wind influence at the
frequency 10 MHz makes additional attenuation about 20 dB at the distance 500
km and the wind speed 15 m/s. It should be noticed that at lower band 3-5 MHz
the additional losses have negative sign. Practically it means the increasing
receiving signal level. Such an effect takes place when an increasing impedance
is reactive, i.e. the sea wave lengths are small while com­paring to radio wave
length [6]. Taking into account the disturbed sea surface losses, the model­ing
of potential HFSWR losses is done both for homogeneous paths (see Fig.4, left)
and mixed paths (see Fig.4, right) using radar frequencies 5, 10 and 25 MHz
marked by dashed-dotted, dashed and solid lines respectively. The utilized data
about HF radar attenuation along dis­turbed sea surface are limited to wind
speed 15 m/s, i.e. sea state code is 7. The influence of sea

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surface under
strong storms and hurricanes is necessary to estimate theoretically and
experimen­tally for distances equal at least the distance to the EEZ's border.

Fig. 4. Two-path surface
wave attenuation while propagating along the disturbed sea surfce, the
homogeneous path (left) and the mixed path (right). The upper curve is at 5
MHz, the lower curve is at 25 MHz. Wind speed is 15 m/s.

Conclusions. The mixed propagation paths bring
additional radar losses for the HFSWR in case of homogeneous patches with
different electric sea and soil properties as well as dis­turbed sea state
influence. The developed software allows predicting and estimating the radar
losses for the mixed propagation paths in real-time operation.

В.А. Зелинский
Научно-технический центр АН ПРЭ 61166, г. Харьков, просп. Ленина, 14, тел
702-18-09 E-mail: akad@kture.kharkov.ua The goal of the given work is review of
modern current state and estimation of future trends in development of air
traffic control radars.